6 
1.15.2.3 
1.15.3 
2.1 
2.2 
2.2.1 
2.2.2 
2.2.4 
The resolution along the field diagonal for at least three lens apertures. 
Where radial and tangential lines are used, data shall be given for both. These 
results shall be obtained for a range of focus settings and the position of best 
average resolution shall established. 
Unambiguous expressions such as "resolving power on radial lines” or “tan- 
gential lines” shall be used in the report. Terms such as “radial” or “radial 
resolving power” shall be avoided as they do not show whether the reference is to 
the direction of the lines or the direction of the resolved separation. 
CALIBRATION 
Calibration should preferably be done photographically under conditions ap- 
proaching closely those which the camera will encounter in service except that 
magazine imperfections are excluded. The temperature should be 20° C. A visual 
method will be permissible if it has been established that it gives the same values 
as the following photographic method to within the required accuracy. 
  
NOTE: Both systematic and random differences exist between laboratory 
distortion measurements and the departures from ideal central projection 
found for a camera in flight. Some causes are curvature of the earth, atmos- 
pheric refraction, camera temperature, air pressure, temperature and pres- 
sure gradients near the camera and aircraft, lack of flatness of the emulsion, 
and dimensional changes in the emulsion. For the most accurate work ac- 
count must be taken of such differences. This can be done from photographs 
of a test area taken with the camera, the aircraft, and the conditions all the 
same as in the survey photography or data on the individual effects for the 
conditions of use can be obtained and combined. 
  
  
  
Definitions 
Fiducial Centre: Point of intersection of fiducial axes. 
Principal Point of Autocollimation: The centre of the image formed in the 
emulsion plane by the camera lens from an incident beam of parallel light which 
in the object space is perpendicular to the emulsion plane. 
  
NOTE: The principal point of autocollimation is the type of “principal 
point” which can be determined most directly and accurately. Other defini- 
tions of principal point have been suggested as partial remedies for inac- 
curate lens centring. If it is desired to use one of them for some particular 
application the test data are sufficient to permit is mathematical determi- 
nation. 
  
  
  
Measured Distortion: Measured distortion is a vector quantity, being the 
displacement from the theoretical image point of an ideal camera of the chosen 
calibrated principal distance to the actual image point for the camera under test. 
It has as components radial measured distortion and tangential measured dis- 
tortion. Radial measured distortion is positive when it is outward from the prin- 
cipal point of autocollimation. Tangential measured distortion is positive when it 
appears as counter clockwise to an observer who is in the image space and is 
looking toward the lens. The origin for distortion measurement is the principal 
point of autocollimation. Measured distortion is zero at that point. 
Theoretical Distortion: Theoretical distortion is an aberration affecting the